This invention relates to an art for taking prompt protection measures against a failure of a switching element that can be caused by an arc surge, etc., and guaranteeing lighting of the discharge lamp connected to the bridge consisting of normal switching elements in a lighting circuit that can light a plurality of discharge lamps.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC-DC conversion circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, to light one discharge lamp, a configuration comprising a DC-DC converter, a full-bridge-type circuit (circuit configured so as to alternately perform on/off control for switching elements making up two arms), and their control circuit can be named. As the switching elements in the DC-DC converter are controlled, output of the DC-DC converter is controlled and AC output provided by the full-bridge-type circuit following the DC-DC converter is supplied to the discharge lamp. Each arm of the full-bridge-type circuit is made up of two switching elements. The connection point of the switching elements making up one arm is connected to one end of the discharge lamp through the starter circuit and an opposite end of the discharge lamp is connected to the connection point of the switching elements making up the other arm.
By the way, when the number of discharge lamps is two, if lighting circuits are provided in a one-to-one correspondence with the discharge lamps for lighting the discharge lamps, it is disadvantageous for the number of parts and the costs and therefore preferably the lighting circuits for the two discharge lamps are made common. For example, the DC-DC converter is configured so as to provide positive output and negative output, the full-bridge-type circuit forming a DC-AC conversion circuit is made up two half bridges, power is supplied to the first discharge lamp through a starter circuit by one arm, and power is supplied to the second discharge lamp through a starter circuit by the other arm. That is, as the switching elements in the DC-DC converter are controlled, output of the DC-DC converter is controlled and when the rectangular wave output provided by the alternation operation according to the switching elements of each of the arms making up the full-bridge-type circuit is supplied to the discharge lamp, positive output and negative output are alternately supplied to each discharge lamp (namely, when positive output is input to one discharge lamp, negative output is input to the other).
However, in the circuit configuration in the related art, if either of the switching elements making up each of the arms of the full-bridge-type circuit fails (for example, when a field-effect transistor is used, short-circuit destruction between drain and source), its anomaly detection is not easy and insufficient or excessive protection measures cause a detriment to occur; this is a problem.
That is, in the configuration in which the full-bridge-type circuit is used to light one discharge lamp, when one switching element fails, if a through state occurs at the output stage of the DC-DC converter, supply voltage rapidly lowers and thus is compared with a predetermined voltage, whereby an anomaly can be detected easily. When it is determined that an anomaly occurs, if an instruction for turning off all switching elements making up the full-bridge-type circuit is issued, expansion of damage or harm caused by the failure of the switching element can be prevented; the situation is easy.
In contrast, in the configuration in which the full-bridge-type circuit is used to light two discharge lamps as described above, if the same concept as that with the number of discharge lamps being one is adopted, an anomaly cannot be detected by simple voltage comparison and sufficient circuit protection cannot be conducted or the detrimental effect accompanying issuance of an instruction for turning off all switching elements (for example, although one discharge lamp and the switching elements of the arm for driving the discharge lamp do not involve any problem, two discharge lamps are switched off because any switching element of the arm for driving the other fails. This introduces a problem as for the driving safety of the vehicle driver if application to the light source of vehicle light is considered; it is preferred to light the one discharge lamp (normal discharge lamp)) is involved. It becomes necessary to determine any of the switching elements making up which of both arms of the full-bridge-type circuit fails and also necessary to make it possible to reliably start the discharge lamp driven by the normal arm.
It is therefore an object of the invention to detect a switching element failure and take necessary and sufficient protection measures in a lighting circuit capable of separately lighting two discharge lamps using a DC-AC conversion circuit of a full-bridge-type configuration.
To the end, according to the invention, there is provided a discharge lamp lighting circuit comprising a DC-DC conversion circuit for receiving a DC input voltage and converting the voltage into positive and negative output voltages and a DC-AC conversion circuit having a plurality of half bridges for receiving the positive and negative output voltages provided by the DC-DC conversion circuit and outputting either positive or negative voltage, wherein power is supplied from each half bridge through a starter circuit to each discharge lamp, wherein an anomaly detection circuit, if the positive and negative output voltages or the output voltages of the half bridges become lower than a predetermined threshold value, for determining that a failure occurs in any of switching elements making up the half bridge.
Therefore, according to the invention, an abnormal state when a failure occurs in any of the switching elements making up the half bridge can be detected reliably.